1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to a monofilament low caliper one-and-a-half layer seamed press fabric for a paper machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
The present invention relates specifically to the press fabrics used in the press section. Press fabrics play a critical role during the paper manufacturing process. One of their functions, as implied above, is to support and to carry the paper product being manufactured through the press nips.
Press fabrics also participate in the finishing of the surface of the paper sheet. That is, press fabrics are designed to have smooth surfaces and uniformly resilient structures, so that, in the course of passing through the press nips, a smooth, mark-free surface is imparted to the paper. Press fabrics accept the large quantities of water extracted from the wet paper in the press nip. Hence, void volume is also important in press fabrics to provide a path for the water to go. The fabric must also have adequate permeability to water for its entire useful life. Finally, press fabrics must be able to prevent the water accepted from the wet paper from returning to and rewetting the paper upon exit from the press nip.
Contemporary press fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a woven base fabric into which has been needled a batt of fine, non-woven fibrous material. The base fabrics may be woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered, multi-layered or laminated. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The woven base fabrics themselves take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a woven seam. Alternatively, they may be produced by a process commonly known as modified endless weaving, wherein the widthwise edges of the base fabric are provided with seaming loops using the machine-direction (MD) yarns thereof. In this process, the MD yarns weave continuously back and forth between the widthwise edges of the fabric, at each edge turning back and forming a seaming loop. A base fabric produced in this fashion is placed into endless form during installation on a paper machine, and for this reason is referred to as an on-machine-seamable fabric. To place such a fabric into endless form, the two widthwise edges are brought together, the seaming loops at the two edges are interdigitated with one another, and a seaming pin or pintle is directed through the passage formed by the interdigitated seaming loops.
Further, the woven base fabrics may be laminated by placing one base fabric within the endless loop formed by another, and by needling a staple fiber batt through both base fabrics to join them to one another. One or both woven base fabrics may be of the on-machine-seamable type.
In any event, the woven base fabrics are in the form of endless loops, or are seamable into such forms, having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross. Because paper machine configurations vary widely, paper machine clothing manufacturers are required to produce press fabrics, and other paper machine clothing, to the dimensions required to fit particular positions in the paper machines of their customers. Needless to say, this requirement makes it difficult to streamline the manufacturing process, as each press fabric must typically be made to order.
Fabrics in modern papermaking machines may have a width of from 5 to over 33 feet, a length of from 40 to over 400 feet and weigh from approximately 100 to over 3,000 pounds. These fabrics wear out and require replacement. Replacement of fabrics often involves taking the machine out of service, removing the worn fabric, setting up to install a fabric and installing the new fabric. While many fabrics are endless, about half of those used in press sections of the paper machines today are on-machine-seamable. Some Paper Industry Process Belts (PIPBs) are contemplated to have an on machine seam capability, such as some transfer belts, known as Transbelt(copyright). Installation of the fabric includes pulling the fabric body onto a machine and joining the fabric ends to form an endless belt.
The present invention is directed to seamed press fabrics. Prior art seamed press fabrics typically consist of two MD yarn layers which form the seam loops in the finished fabric. Even when three MD layers are used, only two of the MD layers are used to form the seaming loops. In these prior art fabrics, various methods are employed to produce loops canted at an angle as perpendicular to the plane of the fabric body as possible. Perpendicular (or vertical) loops ease the process of loop connection and seaming when the fabric is installed in the field.
In cases requiring a low caliper fabric, a single layer seamed press fabric is used. However, with single layer fabric structures, the seam loops cannot be flattened to the caliper of the base fabric. In other words, in order to seam the fabric, the caliper in the loop area must be higher than in the base area. This caliper difference significantly restricts the range of applications for such fabrics since the larger seam caliper may cause operational problems with the paper machine. In addition, single layer structures have a low number of loops per linear area which results in a seam having a relatively lower strength when compared to double layer fabric seams.
Therefore, a need exists for a seamed press fabric for use in applications where a low caliper fabric is desired.
Another aspect of such a seamed press fabric is to provide a sufficiently strong seam while maintaining a caliper similar to that of the main body of the base to prevent seam marking.
A further aspect of such a seamed press fabric is to have a fabric structure that does not collapse under load.
The present invention is a modified endless woven one-and-a-half layer monofilament fabric for use as a press fabric on a papermaking machine. This base fabric provides a solution to the problem of producing a low caliper seamed press fabric.
It is therefore a principal object of the invention to overcome the shortcomings of the fabrics heretofore mentioned.
It is a further object of the invention to provide a press fabric which allows for easy installation and seaming.
Accordingly, the present invention is a papermaker""s fabric for use as a seamed press fabric on a papermaking machine. The fabric has a one-and-a-half layer fabric base of unstacked machine-direction (MD) wefts and cross-machine direction (CD) warps formed by a modified endless weaving process. The MD wefts and CD warps are monofilament yarns. The fabric has seaming loops oriented perpendicular to the plane of the fabric base for easier connection and seaming. The seam loops are formed from adjacent unstacked MD wefts in the fabric base. When the fabric is placed under load, the seam loops result in a seam having a seam caliper substantially similar to a caliper of the base fabric.
Other aspects of the present invention include that the seamed press fabric may be used for applications where a low caliper is desired and for example, where blowing is a problem. The adjacent unstacked MD wefts help to produce a larger xe2x80x9cweb cakexe2x80x9d of needled batt fiber on top of the base than other fabrics having a similar caliper. The seam loops collapse from their perpendicular orientation back into the plane of the fabric base when under load to produce a seam having a seam caliper substantially similar to the caliper of the base fabric. The seam has a sufficient number of seam loops per linear area to result in a seam having relative strength comparable to double layer fabric seams. The MD wefts and CD warps may have a circular cross-sectional shape, a rectangular cross-sectional shape or a non-round cross-sectional shape.